1. Field of the Invention
The present invention relates to data duplicating techniques and more particularly, to a technique effectively applicable to a technique of holding data between a plurality of mutually independent information processing systems in duplicate by using an asynchronous or synchronous data copy technique.
2. Description of the Related Art
For example, in users such as banking organs and insurance companies in which a large capacity of data is held and the loss of data seriously affects the execution of business affairs and social life, taking a backup of the data has hitherto been practiced and besides, in recent years, it has been desired by learning a good lesson from natural calamities and accidents that the data be saved in a backup system laid at a remote location.
More particularly, in order to realize preservation of data and guarantee of operation continuity, update data on a master system is copied to a backup system laid at a remote location and in the event that the master system stops operating owing to a disaster or a fault, the operation is switched to the backup system to ensure continuity of operation.
As regards data preservation based on data duplication between remote locations, a technique disclosed in, for example, JP-A-8-509565 is available. According to the disclosed method, data in a master system is reflected, as it is, on a backup system to maintain a “mirror state” and when operation of the master system is disabled, the operation is switched over to the backup system to thereby make resumption of the system operation easier. The method for backup copy to the master system can be classified principally into two kinds of “synchronous type” and “asynchronous type” from the standpoint of the chance to update the data between the master system and the backup system.
In a technique of the synchronous type, when a request for data update is made by a host of the master system, update data is first written to a storage device of the master system and write of the update data to a storage device of the backup system is then carried out; and when receiving a write end notice from the backup system, the storage device of the master system submits a final write end report to the host of the master system, so that synchronization of the data update between the master system and the backup system can always be maintained.
Contrarily, in a technique of the asynchronous type, when a request for data update is made by the host of the master system, a write end report is submitted to the host of the master system at the time that write of update data to the storage device of the master system is completed but for this data update, execution of data update applied to the backup system is delayed, that is, done asynchronously.
In the case of the asynchronous copy type, a temporary difference in data takes place before the data updated in the master system is reflected on the backup system. Accordingly, a state of the data unreflected on the backup system must be managed or controlled. For example, in a method disclosed in JP-A-10-198607, a difference control table indicating the presence or absence of a difference every logical track is provided and update data is copied to the backup system on the basis of the difference.
In the event that the master system is disabled temporarily owing to, for example, a serious disaster while the “mirror state” being maintained normally, the “mirror state” is sustained by virtue of the synchronousness of data update in the “synchronous type”. In this case, part of data concerning the final update is not reflected possibly on the remote system depending on the timing but matching does not matter. In the “asynchronous type”, however, the update data is reflected asynchronously on the backup system and as a result, a mismatch occurs between the master system and the backup system. In order to recover from the mismatched condition, all of the data in the backup system must be copied to the master system when the master system recovers from the stop state due to a disaster or fault and much time is required for copying a large capacity of data, thus eventually delaying time for the master system to recover.
A method for recovering from the mismatched state is disclosed in, for example, JP-A-6-290125. In the disclosed method, update data is subjected to wait queuing so as to be copied to the backup system in sequence of update, thus maintaining matching. When recovering from a fault, recovery from the mismatched state is made on the basis of a pending write wait queue inside the backup system.
Further, a technique is disclosed in U.S. Pat. No. 5,857,208, according to which in a system for duplicating data between a local system and a remote system, a backup unit such as a magnetic tape device is connected to the remote system, difference data between the local system and the remote system, which difference data is generated when data copy from the local system to the remote system is stopped to copy data to the backup unit at a specified time point, is managed or controlled by a track status provided on the local system side, and after completion of backup by the remote system, the difference data is copied from the local system to the remote system to permit recovery of matching.